The market now may not be tight, but the world's total supply of U-235 is very small. Plus it takes vast amounts of energy to refine it out of the ore, since over 99% of the uranium is U-238. And if I understand the process correctly, it's refined by making it into UF6, which is spun in a chain of centrifuges. Now how do you make UF6? With FOOF! Look that one up... fluorine dioxide. Nasty.
If we really tried to power the world's electric supply with U-235, we'd soon run low. (Or die from meltdowns.) But there's a virtually infinite supply of thorium. It's not just cheap; it's practically free, since it's a waste product of rare earth mining, and we need to refine tons of neodymium in order to have good magnets for motors and generators. Yes, the MSBR needs a seed of U-233, but enough of those reactors do exist.